1. Field of the Invention
The present invention relates to a drive force transmission apparatus disposed between two shafts that are rotatable relative to each other and adapted to transmit rotational torque therebetween.
2. Description of the Related Art
In a conventional drive force transmission apparatus used for four-wheel drive and adapted to transmit torque between front wheels and rear wheels, when an accelerator pedal is depressed and released during a turn on a low-.mu. road, there occurs a phenomenon in which the direction of differential rotation (relative rotation) between the front wheels and the rear wheels reverses.
In order to reduce the size and weight of such a drive force transmission apparatus, there has been proposed a drive force transmission apparatus in which a space filled with a viscous fluid is provided on a side of a piston that presses a multiple disc clutch for torque transmission, and blades are rotatably disposed within the space in order to generate a pressure corresponding to a rotational-speed difference between an input shaft and an output shaft.
In the drive force transmission apparatus, when a rotational-speed difference is produced between the front wheels and the rear wheels, the blades rotate relative to the housing and forcibly move the viscous fluid between two surfaces in close proximity to each other in order to generate a pressure corresponding to the rotational-speed difference by means of the viscous friction effect of the viscous fluid. This pressure acts on the piston so that the multiple disc clutch is brought into a frictional engagement state in order to establish four-wheel drive.
In such a drive force transmission apparatus, since two or three blades are disposed in the circumferential direction such that they project outward in the radial direction, the angle between adjacent blades (inter-blade angel) is large, and therefore the flow range of the viscous fluid is large. Thus, a higher pressure can be generated in response to relative rotation.
However, when the inter-blade angle increases, air contained in the viscous fluid moves within a larger area, with the result that the response speed decreases to some extent when the direction of relative rotation changes.
The response can be improved by a method in which the number of the blades is increased in order to reduce the inter-blade angle, or by a method in which the amount of air mixed into the viscous fluid is reduced to increase the volume ratio between the filled viscous fluid and the space. However, in the former method, the pressure generated in response to relative rotation decreases, and therefore a required torque cannot be transmitted. In the latter method, transmission torque becomes excessively large because of the volume expansion of the viscous fluid stemming from a temperature rise.